Christian is 100% correct. "Your call will be answered in the order it was received" makes no sense. Multiple phone calls can be answered in a particular order, but one phone call can not.
Using coins, as Christian did: Lay three pennies (pretend they are phone calls) down on a table. Pick them up in the same order you laid them down. Easy. Pick them up in a different order than you laid them down. Easy. The concept of "order" makes sense with multiple pennies. Now lay only one penny (pretend it is "your call") on the table. Pick it up in the same order you laid it down. Easy. Pick it up in a different order then you laid it down. It can't be done. To say "your penny will be picked up in the order it was laid down" makes no sense because one penny can only be picked up one way. Only when other pennies get involved ("pennies will be picked up in the order in which they were laid down"), does it make any sense (no pun intended) to mention the order in which they will be picked up.